1. Field of the Invention
The present invention relates generally to high-voltage insulating structures, and more particularly to high-voltage bushings, connectors, and capacitors. The invention particularly relates to (i) high-performance feed-through bushings, (ii) high-performance coaxial connectors, (iii) high-performance multilayer film capacitors that can be operated with high-reliability, and (iv) methods for manufacturing these high-performance components where electric field stresses are eliminated or sufficiently reduced in the so-called weak areas.
2. Description of Related Art
Conventional high-voltage devices such as bushings, connectors, and capacitors use a combination of nonconductive and conductive materials to construct desired high-voltage structures. The nonconductive materials provide a dielectric barrier or insulator between two electrodes of different electrical potential. A wide variety of manufacturing techniques can be employed to construct insulators of the desired shape. Some of the processes that are most often used include machining, molding, extrusion, casting, rolling, pressing, melting, painting, vapor deposition, plating, and other free-forming techniques, such as dipping a conductor in a liquid dielectric or filling with dielectric fluid, etc. The selection process must take into account how one or both of the electrodes made from conductive material will be attached or adjoined to the insulator. The most important factors in producing reliable high-voltage structures are thought to be the quality of the shape of the insulator and purity of the insulator material. Ultimately, what limits the operating voltage of any insulation structure is the intrinsic electrical strength of the dielectric material. In practice, however, the operating levels for typical designs are well below the intrinsic breakdown threshold of the insulating material. The onset of electrical induced failure occurs at the point in the material that cannot support the electric field stress. Electric field stress is defined as a function of voltage and geometric shape. Simply stated, the electric field across two infinite plane electrodes is given by the difference in voltage of the two electrodes divided by the separation distance. This is commonly referred to as the uniform or average field stress. Improvements in the operating levels of a typical design are achieved with the use of impregnating materials. An excellent example of the use of impregnates is vacuum impregnated oil filled capacitors. The air in fabricated capacitors is removed and pressure impregnated with evacuated dielectric oil. Also, an example may involve cables with a coating on a center conductor. Another example involving a semiconductive coating on a highly stressed conductor indicates improvement in higher operating level of previous designs. Both techniques show a marked improvement over nonimpregnated designs, but fall short of being able to operate at th intrinsic strength of the material.
The electric field stresses usually cause the most trouble in the critical regions where dissimilar materials meet, particularly at so-called triple-point regions where a metal electrode and two different dielectric materials are in direct proximity. At these locations, electric field stresses can become severely enhanced, increasing as much as the ratio of the dielectric constants of the different materials, or more, depending on the shapes of the materials. An excellent example of the control used to optimize this effect is described in copending U.S. application Ser. No. 09/034,797, filed Mar. 3, 1998, entitled xe2x80x9cUltra-Compact Marx Type High-Voltage Generator,xe2x80x9d now U.S. Pat. No. 6,060,791 issued May 9, 2000, and assigned to the same assignee. The essence of that invention provides contoured high-dielectric ceramic material and shaped electrodes to control field stress at the triple junction. In contrast, other fabrication techniques may or may not extend electrodes over the edge of straight dielectrics and reduce the operating level of the device. The reduction in operating levels is the result of reaching the intrinsic breakdown level of the weaker dielectric material at lower voltage.
Another problematic area where electric field stresses can lead to failure is in the placement or attachment of electrodes next to solid dielectric materials. Customarily, when the dielectric material is pressed or mechanically fitted between electrodes, there exists small voids that fill with some other material, typically the gas or liquid of the ambient environment in which the high-voltage structure resides. Usually, the material that fills the void has a lower electrical strength than the solid dielectric material, and the electrical field in this highly-stressed region may easily exceed the electrical strength of the void filling material. Moreover, for gas and many liquids having a lower dielectric constant than the solid dielectric, there can be a field concentration in the void region that enhances the likelihood of breakdown in this weaker material.
The present invention overcomes these prior problems by metalizing the surfaces of the solid dielectric materials wherever contact is to be made with metal electrodes. Thus the electric fields are eliminated in the void regions, thereby preventing electrical discharge (corona) activity that can lead to breakdown of the bulk dielectric material. The present invention is similar to the solid-dielectric switch of the above-referenced application, where fields across gaps are removed by metalizing dielectric surfaces. An improvement of high-voltage bushings, connectors, and film-capacitors becomes a natural extension of the above-referenced application.
It is an object of the present invention to remove the electric field from the weaker areas of the internal volume of high-voltage interconnects.
A further object of the invention is to provide for removing electric fields in weak areas of high-voltage bushings, connectors, and cables, such as transmission lines using wedge dielectric structural supports.
A further object of the invention is to provide a practical means for improving the performance of a wide variety of highly stressed insulating structures.
A further object of the present invention is to provide a method for increasing the high-voltage performance of various bushings, connectors, and metal-film capacitors.
Another object of the invention is to provide a conductive surface on the inside surface of the principal solid dielectric insulator surrounding the center conductor and which connects the center conductor to this conductive surface to eliminate electric fields.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention involves removing the electric field from the weak areas of the internal volume of high-voltage interconnects.
The key attribute of the present invention is that it makes use of metalized regions over surfaces of dielectric material that will be attached to or in direct contact with adjoining metal electrodes, thus eliminating any electric field from the void region between the primary dielectric insulation and the adjacent metal electrodes where a weaker dielectric medium may exist.